Modern electronic copiers, printers, facsimile machines, etc. are capable of producing complex and interesting page images. The pages may include text, graphics, and scanned or computer generated images. The image of a page may be described as a collection of simple image components or primitives (characters, lines, bitmaps, colors, etc.). Complex pages can then be built by specifying a large number of the basic image primitives. This is done in software using a page description language such as POSTSCRIPT™. The job of the electronic printer's software is to receive and interpret each of the imaging primitives for the page. The drawing, or rasterization must be done on an internal, electronic model of the page. All image components must be collected and the final page image must be assembled before marking can begin. The electronic model of the page is often constructed in a data structure called an image buffer. The data contained is in the form of an array of color values called pixels. Each actual page and the pixel's value provides the color which should be used when marking. The pixels are organized to reflect the geometric relation of their corresponding spots. They are usually ordered to provide easy access in the raster pattern required for marking.
In the prior art, a copier, printer or other document-generating device typically employs an initial step of charging a photoconductive member to substantially uniform potential. The charged surface of the photoconductive member is thereafter exposed to a light image of an original document to selectively dissipate the charge thereon in selected areas irradiated by the light image. This procedure records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. The latent image is then developed by bringing a developer material including toner particles adhering triboelectrically to carrier granules into contact with the latent image. The toner particles are attracted away from the carrier granules to the latent image, forming a toner image on the photoconductive member, which is subsequently transferred to a copy sheet. The copy sheet having the toner image thereon is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
The approach utilized for multicolor electrophotographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color corresponding thereto and the process is repeated for differently colored images with the respective toner of corresponding color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
With the increase in use and flexibility of printing machines, especially color printing machines which print with two or more different to colored toners, it has become increasingly important to monitor the toner development process so that increased print quality, stability and control requirements can be met and maintained. For example, it is very important for each component color of a multi-color image to be stably formed at the correct toner density because any deviation from the correct toner density may be visible in the final composite image. Additionally, deviations from desired toner densities may also cause visible defects in mono-color images, particularly when such images are half-tone images. Therefore, many methods have been developed to monitor the toner development process to detect present or prevent future image quality problems.
For example, it is known to monitor the developed mass per unit area (DMA) for a toner development process by using densitometers such as infrared densitometers (IRDs) to measure the mass of a toner process control patch formed on an imaging member. IRDs measure total developed mass (i.e., on the imaging member), which is a function of developablitiy and electrostatics. Electrostatic voltages are measured using a sensor such as an ElectroStatic Voltmeter (ESV). Developability is the rate at which development (toner mass/area) takes place. The rate is usually a function of the toner concentration in the developer housing. Toner concentration (TC) is measured by directly measuring the percentage of toner in the developer housing (which, as is well known, contains toner and carrier particles).
As indicated above, the development process is typically monitored (and thereby controlled) by measuring the mass of a toner process control patch and by measuring toner concentration (TC) in the developer housing. However, the relationship between TC and developability is affected by other variables such as ambient temperature, humidity and the age of the toner. For example, a three-percent TC results in different developabilities depending on the variables listed above. Therefore, in order to ensure good developability, which is necessary to provide high quality images, toner age must be considered.
Consequently, there is a need to provide a method and apparatus for calculating or determining toner age to ensure image quality by anticipating or diagnosing problems in image quality, which may be caused by toner age. These problems include low developability, high background, and halo defects appearing on sheets of support material. One method of managing the residence time of toner in the developer housing is to use a minimum area coverage (MAC) patch in the inter-page zone to cause a minimum amount of toner throughput which is disclosed in U.S. Pat. No. 6,047,142 which is hereby incorporated by reference. However there is a drawback with this solution in that toner throughput is increased resulting in raising the consumables cost and Total Cost of Ownership (TCO) of the system. Thus minimizing the excess toner throughput is important for print shop cost control.